Radiant energy apparatus



SR Ems EEEERENEE sEARcH ROOM 392879735 BIG-121- MTRIM Nov. 22, 1966 H.R. DAY, JR

RADIANT ENERGY APPARATUS 5 Sheets-Sheet 1 Filed Aug. 28, 1962 1 7'? v en t2: 2-":- /-/& ro/d 7?; Day a; 00 2 W #:t'arrvey.

Nov. 22, 1966 H. R. DAY, JR

RADIANT ENERGY APPARATUS 5 Sheets-Sheet 2 Filed Aug. 28, 1962 [r7 vervtor: Harold Rud u Ator'ney.

Nov. 22, 1966 H. R. DAY, JR 3,237,735

RADIANT ENERGY APPARATUS Filed Aug. 28' 1962 5 Sheets-Sheet 5 2inventor: 25 J Hare/d IQDdy c/ifj Attorney.

United States Patent 3,287,735 RADIANT ENERGY APPARATUS Harold R. Day,Jan, Burnt Hills, N.Y., assignor to General Electric Company, acorporation of New York Filed Aug. 28, 1962, Ser. No. 220,011 16 Claims.(Cl. S M-74) This invention relates to the production of radiationwithin a closed chamber for effective utilization outside such chamber,and more particularly to the effective transmission of focused electronbeams through an electron permeable closure onto a recording medium.

The apparatus according to the present invention finds particularutility in the production of diffraction phase gratings on a deformablemedium such as a thermoplastic material. The general method, apparatusand medium are described and claimed in the copending application ofWilliam E. Glenn, In, Serial No. 8,842, filed February 15, 1960, nowPatent No. 3,113,179, a continuation-inpart of application Serial No.698,167, filed November 27, 1957 (now abandoned), and of applicationSerial No. 783,584, filed December 29, 1958 (now abandoned), allassigned to the assignee of the present invention. According to thissystem of recording, diffraction gratings are inscribed on thedeformable thermoplastic medium with a controlled electron beam a fewmicrons in diameter. Areas where electron charge is deposited by thebeam are compressed due to electrostatic attraction between the surfacesof the recording medium, causing the formation of diffraction phasegratings. After the deformable medium is thus impressed with the desiredintelligence, the deformable medium may be placed in a projectionapparatus which blocks non-diffracted light while projecting an image ofthe intelligence recorded in the gratings. The recorded gratings maycontain picture information such as, for example, a television image.

During diffraction grating recording, both the deformable medium and theelectron gun forming the electron writing beam are conventionally housedin the same evacuated chamber for two reasons: (1) to maintain a long,mean-free path for the electrons so they dont suffer dispersion by toomany collisions with gas molecules, and (2) to protect the cathode orfilament of the electron gun from erosion by, and reaction with, gasmolecules. The second reason is more pressing. However, it is difficultto maintain a vacuum under recording conditions because the recordingmedium produces gaseous products when bombarded with electrons. Ittherefore appears the electron gun and recording medium are to a largedegree incompatible. The result is short cathode life and low cathodeefiiciency.

Previous attempts to solve this problem by separating the electron gunand the recording medium as with an intervening electron permeablewindow have not been successful; therefore, diffraction recording inthis manner has been considered impractical. Metal and metal oxidewindows of the type employed, for example, in electron irradiating andsterilizing apparatus cause the electron beam to be highly scattered andabsorbed, presumably due to collision of electrons with atoms of thewindow. The beam is then much too large to be used to form diffractiongratings. For example, a beryllium metal window having a thickness onthe order of 0.001 inch, or somewhat less, will scatter an electron beamto a diameter of several mils, and the resulting beam will have anindistinct edge. This window thickness, required to prevent pinholes,also necessitates use of a hundred kilovolt magnitude electron beam forpenetration.

The scattering caused by a window has been determined heretofore on astatistical basis relative to the number of atoms in the window, andscattering is predicted for any type or thickness of window. In passingthrough the conice ventional window, the electrons are found not only toscatter in direction but to change in velocity so that refocusing of thebeam is impossible. Furthermore a photographic plate, even though placedimmediately adjacent a conventional window, reveals impracticablescatte-ring which is not noticeably improved by focusing the beam at theplane of the window.

I have discovered that certain extremely thin windows, particularly whenformed of atomically highly oriented material, produce little or noscattering as statistically predicted, but rather pass an electron beamor other radiant energy beam with surprisingly high definition. When thewavelength of the radiant energy beam is small compared to theinter-atomic spacing in thin windows, and particularly when a windowsatomic construction is comparatively regular throughout its thickness,an increasing number of beam particles must be considered as passingthrough the window on a quantum mechanical basis without effectivelysuffering collision or scattering. These unscattered particles arecoherent, that is unchanged in direction and velocity, making itpossible to maintain focus or produce focusing after the beam haspenetrated the window.

In accordance with various features of the embodiments of my inventionherein illustrated, a thin electron permeable window is disposed, forexample, between the cathode and the beam focusing and deflecting meanslocated in a chamber'separate from the cathode. A particularlyadvantageous placement of the thin window is across a small aperture inan electron gun anode or other gun electrode. The recording medium, nowlocated in the chamber remote from the cathode, will not contaminate thecathode. The chamber including the recording medium is desirablyevacuated to provide a long meanfree path for electrons, but suchevacuation need not be nearly so exacting as the vacuum in the chamberhousing the electron emitting cathode. The gaseous products from therecording medium are easily tolerated in the additional chamber.

In accordance with another feature of the present invention, the thinelectron permeable window is formed of highly oriented pyrolyticgraphite or similar material having a regular or nearly monocrystallinestructure. A Window of such material passes a high percentage of anelectron beam or other radiant energy beam and does so withoutdetectable scattering.

The subject matter which I regard as my invention is particularlypointed out and distinctly claimed in the concluding portion of thisspecification. The invention, however, both as to organization andmethod of opera tion, together with further advantages thereof, may bestbe understood by reference to the following description taken inconnection with the accompanying drawings wherein like referencecharacters refer to like elements and in which:

FIG. 1 is a side view partially broken away of an electron-beam,light-valve apparatus constructed in accordance with the presentinvention.

FIG. 2 is an enlarged broken away view, partially in cross-section, ofthe chamber including an electron gun and electron permeable exit windowin accordance with the present invention,

FIG. 3 is an enlarged cross-sectional view of an electron gun anode andelectron permeable window,

FIG. 4 is a plan View of an alternative construction according to thepresent invention wherein electrostatic focusing is employed,

FIG. 5 illustrates an electron permeable window mounted upon aninsulated support ring, and

FIG. 6 illustrates an electron permeable window mounted on a thinmetallic ring.

The electron beam apparatus as illustrated in FIGS. 1,

2 and 3 employs an electron permeable vacuum-tight window 1 of less than500 Angstroms in thickness and preferably on the order of 50 to 200Angstroms in thickness. This window separates first and seconddifferentially-evacuated, glass-walled regions or chambers 2 and 3, theformer including an electron gun at 4 for producing an electron beam 5directed through the Window 1. The window is desirably formed ofmaterial Which is highly oriented atomically and has a regular or nearlymonocrystalline structure. Pyrolytic graphite is preferred. This type ofmaterial, while providing an effective barrier to gas molecules, isbelieved to present a screen-like structure to an approaching electronbeam whereby a well defined central electron beam readily passes throughthe materials atomic structure without the usual scattering.

The electron gun 4 of the present embodiment comprises a tungstenfilamentary cathode 6, a grid electrode 7 and an accelerating anode 8.The cathode and grid are adjustably supported in chamber 2 and may bepositioned by means of adjusting screws 9. Anode 3 is conductivelysupported on a centrally apertured anode plate 10. For purposes of thepresent invention, plate 10 is considered as permanently secured acrossthe bottom of chamber 2 forming a gas-tight seal therewith. However,plate It is separable from chamber 3 as by loosening the bolt 11 whichnormally secures the chambers together.

The anode 8 supports the gas-tight but electron permeable window 1, thelatter being disposed across a small central beam defining aperture 12in the anode. The electron beam is narrowly focused and restricted ataperture 12, which is desirably on the order of 2 to 10 mils indiameter. Grid 7 is maintained negative with respect to the electronbeam (by means not shown), and anode 8 is maintained quite positive withrespect to the electron beam (by means not shown); the electric fieldproduced by these two electrodes acts to focus the beam at aperture 12,provided the electrode voltages are properly adjusted in a manner wellknown by those skilled in the art. The aperture being small tends torestrict the electron beam, reducing aberration effects.

Several advantages accrue in locating the electron permeable windowacross this small aperture. The window, while retaining physicalstrength, may be substantially thinner when disposed over a very smallaperture than would be the case if the window were located at some otherpoint in the system across a larger aperture. The window then passes alarge percentage of the electrons impinging thereon as an unscatteredcentral electron beam, because of the thinness, and also because thebeam velocity is at a peak at the positive anode. For example, anannealed pyrolytic graphite window, 200 Angstroms thick located at thispoint passes approximately 50% of a 15 kilovolt electron beam. Theremaining 50% of the beam is not randomly scattered; separate anddistinct diffraction images are produced which are radially separatedfrom the central beam and account for the remaining 50%. Thesediffraction images are intercepted by a metal diaphragm 13 which islocated downstream from the window 1, while the non-scattered centralbeam passes through an aperture 14, larger than aperture 12, locatedcentrally of the diaphragm 13.

It is of course understood that other electron gun structures may beemployed in place of the one herein illustrated. It is however generallydesirable to place the electron permeable window across the smallestaperture through which the electron beam passes and where the beam has ahigh velocity as compared with elsewhere in the same apparatus. Thiswill generally be at a crossover or focus point for the electron beam.

The second region or chamber, generally designated at 3, which nowreceives the electron beam, includes a transparent charge deformablemedium 15. This medium may comprise a deformable oil of the type setforth in Patent 2,943,147, to William E. Glenn, Jr., issued June 28,1960, and assigned to the assignee of the present invention, or athermoplastic material as hereinafter described. The deformable medium15, conveniently supported in a suitable light transmitting holder 16,is positioned in the path of electron beam 5. The electron beam causeselectrostatic deformation in the medium for forming light-diffractingphase gratings therein.

Magnetic focusing coil 17 and deflection coils 17a act upon the electronbeam 5 to produce a finely focused electron beam at the surface of thedeformable medium 15, while the electron beam is deflected in a patternsuch as that of a television raster. As the electron beam is deflectedalong a raster line, it is also influenced by deflection coils 17a in amanner to momentarily slow, speed up, and even halt the deflection ofthe electron beam along a raster line to cause varying charge densityalong the line where the beam impinges upon the medium 15. The varyingcharges produce deformations in the medium in the form of diffractionphase gratings capable of dilfracting light. This manner of deflectionfor establishing phase gratings containing three-color televisioninformation, for example, is set forth in Patent Re. 25,169, to WilliamE. Glenn, Jr., which is assigned to the assignee of the presentinvention.

To provide read-out of the foregoing information, light from a lightsource 26 is concentrated for illumination of medium 15 by means of acondensing lens 18. Another lens 19 normally images a plurality ofoptical slits 2b, which are illuminated by source 26, upon a pluralityof opaque bars 21. However, when diffraction intelligence is impressedon the medium 15, the phase gratings thus formed will ditfract the lightfrom light source 26 so that diffracted light passes between the bars 21through slits 22. A lens system 23 then acts to image the intelligencecontained in the diffraction gratings upon a screen 24. Mirror 25 isincluded in the light path between lens system 23 and the screen, forconveniently providing a vertical image of the diffraction intelligencethereon.

The electron beam 5 is thus focused and deflected, in accordance withone feature of the invention, after having passed through electronpermeable window 1. This is made possible because in passing through theextremely thin electron permeable window 1, the beam is substantiallyunaltered in direction and velocity, in contra-distinction to priorelectron permeable windows. Thus a microscope inserted in the FIG. 1apparatus at a glass window 2'7 reveals no detectable change in thecross-section of the electron beam. When a photographic plate issubstituted for medium 15, and the electron permeable window 1 isalternately removed and reinserted, no change in the beam image isdetected, except in its intensity.

Focusing coil 17 produces concentration of the electron beam into asmaller spot size at the deformable medium 15. For example, a beam onemil in diameter at aperture 12 can be concentrated by focusing coil 17to a less than M; mil diameter at deformable medium 15, a size quiteappropriate for establishing diffraction gratings. The focusing actionproduced may in the alternative be considered one of refocusing theelectron beam or imaging aperture 12 upon medium 15. Without suchfocusing the beam would have be come much larger downstream, with orwithout a window.

It is to be chiefly noted in this connection that an electron beampassing through electron permeable windows, as heretofore known, couldnot be further focused or concentrated mainly because the beam electronswere randomly altered in velocity in passing through the window. Thus afocusing field appropriate to the velocity of some particular electronwould not be appropriate for other electrons of the beam, thus causing abeam which could not be refocused. In the present invention, however,further focusing is as easily accomplished in the same manner as if theelectron permeable window were absent.

The same phenomena are noted with regard to deflection. That is,electrons having varying velocities would be deflected to differentdegrees by a given deflection field. However, in accordance with thepresent invention, deflection in region 3 is readily and accuratelyaccomplished because the beam electrons have not been differentiallyaltered in velocity.

During operation of the apparatus, electron gun 4, isolated in chamber2, remains unaffected by gaseous products produced outside this chamberwhen, for example, the electron beam strikes medium 15. The medium 15comprising recording oils or thermoplastic material is found to have theproperty of evolving a carbonaceous oily vapor and gas ions when heatedand subjected to electron bombardment. In accordance with the presentinvention, these products cannot reach the cathode or the gun electrodesand therefore form no objectionable coatings thereon. These gaseousproducts cannot then poison a cathode by chemical combination therewith,nor are deleterious dielectric layers deposited on the other electrodeswhich would alter their electron-optical characteristics. It is easilypossible to maintain a good vacuum on the order of mm. Hg or less in theregion 2 which includes cathode, while at the same time permitting alesser vacuum of only 10- mm. Hg to lO mm. Hg, for example, in chamber3.

The carbonaceous deposits which would otherwise form upon the electrodesof the electron gun will now have a tendency to deposit upon theelectron permeable window 1. While such deposits do not have the samedebilitating effect upon the window, it has been found advantageous tokeep the window free of such substances. An additional reason, then, forplacing the electron permeable window at the beam defining aperture 12,where the electron beam is most restricted, is the heating produced inthe window at this point. It is found this heating desirably producesevaporation of carbonaceous oil deposits from the window surface. Thegas ions produced in region 3 which would have a tendency to bombard thecathode in the ordinary apparatus, also cannot reach the cathode becauseof the interposition of window 1.

The cathode 6 may be run at a lower temperature than heretofore foundnecessary while still producing an acceptable electron emission, andtherefore its lifetime is multiplied several times over. The tungstenfilamentary cathode in the drawing is illustrative only and isreplaceable with higher emission cathodes, for example, of the alkalineearth metal oxide type. Such cathodes produce copious electron emission,but have heretofore been completely incompatible with the carbonaceousdeformable medium located in the same apparatus.

An alternative embodiment of the present invention employingelectrostatic focusing, and wherein the electron permeable window issupported, for example, on an insulating ring, is illustrated in FIG. 4.In the embodiment of FIG. 4, an electron gun comprises a cathode 6having an electron emissive coating at 7 which may comprise aconventional thermionically emitting oxide. The cathode is internallyheated by means of a separate filament 28. A grid 29, negative withrespect to the cathode, includes an aperture 30 through which electronbeam 5 passes. The electron beam is directed through an aperture 31 in acylindrical anode electrode 32, anode 32 forming together with annularmetal spacer 33 the end of the region or enclosure 2.

The aperture 31 is covered with a vacuum tight but electron permeablewindow 1 supported upon the anode with a refractory ceramic insulatingspacer 34. The optics of the electron gun system are arranged such thatan electron beam crossover or focus occurs at the window 1. The heatingproduced by the electron beam at the electron permeable window 1,preferably formed of pyrolytic graphite, is conserved to the Windowthrough the use of an annular ceramic spacer 34 shown in detail in FIG.5. Thus the electron permeable window tends to operate at a hottertemperature and more readily evaporates any gaseous products which maybecome deposited thereon. An alternative window supporting constructionis illustrated in FIG. 6 wherein electron permeable window 1 is sup- 6ported on a thin metallic annular member 35 which may be substituted forthe ceramic member 34 in the FIG. 4 apparatus. The metallic annularmember 35 is conductive but is preferably formed of a material thin incomparison to anode 32 so that it tends to conserve heat to the window1.

Returning to FIG. 4, the electron beam 5 passing through window 1 isfurther focused and concentrated by means of an electrostatic lensincluding electrodes 36, 37 and 38. Again, it should be noted theelectron beam in passing through the window has received essentially noscattering and therefore may receive further focusing and deflection.The electrostatic focusing system comprising electrodes 36, 37 and 38acts to focus and concentrate the electron beam 5 to a spot size on theorder of 5 microns upon a thermoplastic tape 40 where the tape passesover capstan 41.

A pair of electrostatic deflection plates 39 deflect the beam in adirection transverse to the movement of tape 40 between tape reels 42and 43. A television type raster may then be formed by moving the tapefrom reel 42 onto reel 43 as the electron beam is deflected transverselyof the tape. This transverse deflection may thus be velocity modulatedin order to cause the formation of a charge pattern on the tape 40. Thethermoplastic tape is heated by a heating means 44 before reachingcapstan 41 so that in its softened condition it is capable ofdeformation in response to the charge pattern to develop diffractionphase gratings thereon.

The thermoplastic tape 40 is of the type set forth and claimed in theaforementioned application of William E. Glenn, ]r., Serial No. 8,842.Briefly this tape comprises a base material carrying a thermoplasticcoating which coating is oriented towards the impinging electron beam.The base material is optically clear and smooth and may suitablycomprise 4 mil thick optical grade polyethylene terephthalate sold underthe name Oronar. The thermoplastic layer on the film is also opticallyclear, having a substantially infinite room temperature viscosity and arelatively fluid viscosity at a temperature of 100-150 C., to whichtemperature it is heated by means 44. One satisfactory thermoplasticmaterial is a blend of polystyrene, m-terephenyl and a copolymer ofweight percent of butadiene and 5 weight percent styrene. Specifically,the composition may be 70 percent polystyrene, 28 percent m-terephenyland 2 percent of the copo-lymer. The film thickness can vary from about0.01 mil to several mils, with the preferred thickness being about equalto the distance between desired depressions in the film for forming thephase gratings, e.g. approximately 5 microns or approximately 0.2 mil inthe present instance.

The operation of the FIG. 4 apparatus is quite similar to that of theFIG. 1 apparatus and need not be recited in detail. The electronpermeable window 1 passes a central beam 5, as illustrated, and alsotends to produce a number of electron diffraction images which areconveniently intercepted by diaphragms 45 and 46 of lens electrodes 36and 38. These diaphragms are provided with centrally located apertures47 and 48 for passing the central electron beam 5. Again, no observablescattering of the central beam takes place. Assuming a 10 kilovoltelectron beam, electrodes 36 and 38 may be conveniently provided with avoltage of 8 kilovolts while electrode 37 is provided with a voltage ofapproximately 3 kilovolts. It is understood these voltages are exemplaryonly.

The electron permeable window may be disposed across either aperture 47or 48 as an alternative to placement thereof across aperture 31 in theanode electrode. If the electron permeable window is placed acrossaperture 47 then diaphragm 46 will prevent passage of the outlyingdiffracted emissions. If the window is placed at aperture 48 anadditional aperture (not shown) may be provided for stopping theadditional r-ays. It is quite desirable from a standpoint of passing asmuch of the central electron beam as possible, that the window be placedacross a small aperture in a high potential electrode where the electron'beam attains a high velocity relative to other portions of the tube.Such electrode will frequently be a high positive voltage electrode butits actual physical location may vary from one electron beam apparatusto another. Frequently the high voltage electrode will be the last orexit electrode in what may be considered the electron gun or electronoptical system. The window may also be secured on a separately providedapertured support in either the FIG. 4 or FIG. 1 apparatus.

Although the thin electron permeable window in accordance with thepresent invention may be formed from various materials, pyrolyticgraphite, as a highly oriented and semi-single crystalline material,possesses numerous advantages and is generally preferred. The pyrolyticgraphite, in addition to being an atomically regular material, isresistant to high temperatures and to oxidation. It also is mechanicallystrong and essentially gas tight. The pyrolytic graphite as employed forthe electron permeable Window according to the present invention may bedefined as a material made from carbonaceous gases by thermaldecomposition or formed of carbonaceous material by evaporation anddeposition on a surface. Briefly, a hydrocarbon gas, such as methane, isdeposited on a surface heated to the range of 1800 to 2500 C. in achamber wherein the carbon gas pressure may vary between 0.5 mm. and 760mm. of mercury. The heated surface upon which the pyrolytic graphite isdeposited is also preferably formed of a graphite substance.

Prior to admitting the hydrocarbon gas, such chamber is evacuated ofother gaseous materials. The carbonaceous gas is then decomposed to acarbon vapor which deposits upon the heated surface. The graphite bodydeposited at these temperatures is a fine grained pyrolytic graphitewhich is quite free of unusually large gasnucleated particles. Afterdeposition of the pyrolytic graphite, the surface with the graphitedeposed thereon is allowed to return to room temperaure.

The body of pyrolytic graphite thus formed is preferably annealed at atemperature of at least 3500 C. and preferably in the range of 3500 to3800 C. The annealing is carried out in a suitable enclosure in thepresence on an inert gas such as argon which is desirably circulatedthrough the enclosure. This anneal results in pyrolytic graphite withsuperior crystalline perfection and preferred orientation and results innearly complete straightening of the planes of graphite forming thematerial. The pyrolytic graphite as herein described is more fullydescribed and claimed in the patent application of Russell J.Diefendorf, Serial Number 199,467, filed June 1, 1962, and assigned tothe assignee of the present invention.

A thin layer of pyrolytic graphite, suitable for an electron permeablewindow, is then separated from the above formed graphite body. Onemanner of securing extremely thin flakes of pyrolytic graphite materialis to immerse the material in benzene while peeling thin layerstherefrom with a needle under a microscope. The graphite tends toseparate into thin planes possibly formed of a number of flatcrystallites and useable as Window material. A method found moreeffective is the separation of the graphite material using a pressuresensitive adhesive, for example, ordinary cellophane tape having apressure sensitive adhesive layer. The cellophane tape is impressed upona body of pyrolytic graphite material and then stripped therefrom. Athin layer of graphite will adhere to the adhesive layer. A second pieceof tape is then impressed upon the graphite material carried by thefirst tape. The tapes are then stripped from one another. An eventhinner layer of graphite now adheres to the pressure sensitive adhesiveof each tape. A fresh tape is then applied to one of these layers andthen stripped off again resulting in a yet thinner layer of pyrolyticgraphite. This procedure is continued until a graphite layer is securedwhich is readily transparent. The thickness of the graphite layer can begauged with an interferometer or with a light absorption meteringdevice, but with practice the indication of light transparency to theeye is usually sufficient for determining the proper graphite layerthickness, under 500 Angstroms and preferably less.

The adhesive material is now dissolved by immersing the cellophane tapeincluding the attached graphite layer into a solvent solution. Atypically satisfactory solvent for the self-adhesive layer onconventional cellophane tape is an equal part mixture of toluene (C H CHpyridine (C H N) and chloroform (CHCl The pyrolytic graphite flakes arefound to separate freely from the tape in this solution.

In preparation for the implacement of the pyrolytic graphite window, theaperture across which the window is to be disposed is now drilled with avery small diameter hole. Anode 8 illustrated in FIG. 3, and for exampleformed of stainless steel, is drilled with a perpendicular hole 12, 5l0mils in diameter. The hole is cleaned and polished so as to be burrlessespecially in the direction which will subsequently adjoin chamber 3,i.e., the surface upon which the window 1 will rest. The anode is thensecured to anode plate 10, and the entire chamber 2 including theelectron gun is oriented with the anode end of the gun upright. Thechamber 2 is attached to an evacuation means through a tip off valve(not shown) causing an inflow of air through the small aperture 12. Nowa small flake of pyrolytic graphite is transported from the solvent on afine mesh screen and, with the aid of microscope means, is disposed uponthe aperture 12. The fine mesh screen is removed. The evacuation meanswill cause the pyrolytic graphite layer to bow downwards slightlythereby insuring its continued implacement. However, the small pyrolyticgraphite window is found to adhere substantially permanently to theanode structure even if the vacuum in region 2 is discontinued. Theevacuation of region 2 can now be completed.

It is postulated the comparative transparency of pyrolytic graphitematerial to a beam of electrons is due to its regular atomic structure.In pyrolytic graphite, planar graphite crystallites have a preferredorientation, and are arranged so their layers are generally parallel tothe body suface or the surface upon which they were deposited. Theannealing of a pyrolytic graphite body at a temperature above 3500 C.results in an article of superior crystalline perfection and superiororientation. The annealing appears to result in complete straighteningof the grains of the material in planes substantially parallel to thesurface, to the extent of a nearly perfectly oriented structure. Theorientation of the resulting material is effectively quite similar to asingle crystal material. As far as the electron optics of the materialare concerned, longitudinal metal grain boundaries which appear todivide most metals into a mosaic of variously oriented crystalstructure, are either not present in the pyrolytic graphite as annealed,or else such boundaries are so substantially infrequent as not to affectthe electron optics deleteriously. Seen under a microscope, smallcrystalline units of the graphite at first appear to become larger andthen the boundaries appear to nearly vanish in annealing. To the eye,the body of pyrolytic graphite appears as a mirror and behaves otherwiseas a single crystal.

In placing a window of such material in an apparatus, such as theapparatus of FIG. 1, double images sometimes occur for some windowpositions and beam orientations, indicating the possible presence of agrain boundary. However a very slight reorientation of the window or ofthe electron beam easily eliminates one of these images, and it isthought that the electron beam then panatrates the window withoutencountering such a grain boundary.

The graphite window flake is considered as being composed of one or moreplanar crystallite layers the planes of which are substantiallyperpendicular to the direction of the electron beam. The electron beamimpinging upon the thin layer of crystalline material appears to passstraight through as a central beam of somewhat reduced intensity. Theremaining beam is not scattered randomly near the central beam, but isdiffracted by the atomic planes of the window material. This electrondiffraction produces electron beam images at points radially separatedfrom the central beam for example as viewed upon medium 15. The electrondiffraction images are found to be as distinct as the central beam butradially displaced sufficiently from the central image so they may beeasily intercepted by diaphragm 13. The central beam, which itselfexhibits no scattering effects, passes through aperture 14 and thediffraction images are intercepted by the diaphragm 13.

The electron beam preferred according to the present invention is a lowvelocity beam as compared with most prior apparatus wherein a very highvelocity electron beam has been required in order to pass through awindow. Thus the electron beam in the instant apparatus has an energy onthe order of 10,000 electron volts, this energy being quite sufficientfor producing central electron beam penetration of 50% through theextremely thin pyrolytic graphite. Very high energy beams (50 100 kv.)are not only unnecessary according to the pres ent invention butfrequently tend to have undesirable effects upon deformable media.

At the electron beam energies herein indicated, the electrons making upthe beam are considered to have a wavelength appropriate for passingthrough the graphite crystalline structure without scattering. The dualnature of an electron-having wave properties as well as corpuscleproperties-is well known in quantum mechanics. The wavelength of anelectron is related to the velocity of an electron by the formula:

A=wavelength h=Plancks constant m=electron mass, and v=velocity of anelectron The electrons in an approximately kilovolt beam will be foundto have a wavelength which is short as compared to the inter-atomicspacing of a metal such as pyrolytic graphite. The complete beam ofelectrons of course has a very large diameter compared to thisinteratomic spacing of the window material, but may be looked upon aspassing through the essentially singular crystalline atomic structure asthrough an atomic mesh screen or a properly oriented honeycomb. If theelectron beam has low energy, on the other hand, the electron wavelengthis comparable to the inter-atomic spacing and appears not to produceready penetration of the window material. However, at extremely lowelectron velocities, e.g., as obtained by thermionic emission with nosubstantially accelerating electrodes acting upon the beam, readypenetration of the window is again observable. Thus, a thermionicfilament o-r cathode placed in the region 2 of the illustrated apparatuswill produce electrons capable of easily passing through the thinelectron permeable window. In such an instance, the window itself may beutilized as an essentially cold cathode emitting surface but oneproviding a copious supply of electrons. Such an emitting surface isuseful in many different discharge type devices. The ready penetrationof the low velocity electrons through the window takes place when theelectron has a long wavelength, compared to the inter-atomic spacing inthe window. First order diffraction, if it takes place, is so fardisplaced radially as to be unobservable. It thus appears thatpenetration can take place for electron wavelengths which substantiallydiffer from the inter-atomic spacing of the crystalline window material.

Although pyrolytic graphite is preferred as a window material, it isunderstood the invention in all its aspects is not limited to pyrolyticgraphite. Other substances are also suitable to varying degrees whenreduced to extreme thinness. These materials, while frequently resultingin some scattering, .do produce surprisingly good results in thicknessesbelow 500 Angstroms and preferably below 200 Angstroms. In suchthicknesses such windows produce much less scattering than would bepredicted. Thus aluminum oxide (A1 0 produces surprisingly littlescattering as employed in an extremely thin electron permeable window.One disadvantage, however, of the aluminum oxide or alumina is itssusceptibility to the intense heat caused by the electron beam due tothermally insulating nature of the material. For this reason it issometimes desirable when using an alumina window to mount such windowbeyond the beam focal point or crossover in order to reduce heating.Aluminum oxide material of which a thin window is formed may be procuredby anodic oxidation and obtained in a self-supporting film as describedin an article by Louis Harm's, Journal of the Optical Society ofAmerica, vol. 45, No. 1, page 27.

Other materials which are suitable to varying degrees include pyrolyticboron nitride and tantalum oxide. Pyrolytic boron nitride, for example,has a regular atomic structure as compared to many metals, metals ingeneral providing relatively poor electron transmission because of theirusual heterogeneous internal structure. The preferred materials are ingeneral somewhat refractory and their elementary constituents have a lowatomic number, i.e. carbon, nitrogen, boron and aluminum, electronabsorption increasing with higher atomic numbers. Evaporated carbon inextremely thin layers is sometimes satisfactory.

According to the illustrated embodiments of the present invention,electron beams have been described as passing through a thin windowarrangement. However, in its broader aspects the present inventioncomprehends passage of other forms of radiant energy through anextremely thin radiant energy permeable window. For example X- rays arefound to pass through thin windows according to the present inventionwith little or no central beam scattering.

In accordance with the present invention as above stated, numerousadvantages are secured. An electron gun or other radiant energyproducing means can be accommodated in its own isolated and sealed offregion whereby the electron beam passing through the permeable window isdirectable in a second chamber, and may be focused therein to a veryhigh degree. The second region may, if desired, constitute theatmosphere, within the limits dictated by air scattering and ionization.The electron gun or other radiant energy producing means, beingisolated, is not subjected to cathode poisoning by gaseous productsfound in the second region. The radiant energy generating means will,moreover, not be deteriorated by ion bombardment from the second region.Also, the various electrodes in the first region are not subjected todeposition of foreign dielectric material and the like and thereforetheir electrical properties are not impaired with age.

While I have shown and described several embodiments of my invention, itwill be apparent to those skilled in the art that many changes andmodifications may be made without departing from my invention in itsbroader aspects; and I therefore intend the appended claims to cover allsuch changes and modifications as fall within the true spirit and scopeof my invention.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. A recording apparatus comprising a deformable recording medium, meansfor heating said medium, an electron gun producing a narrowly focusedelectron beam directed towards said medium for recording diffractionphase gratings on said medium, and an evacuated chamber including saidelectron gun and provided with a window of thin crystalline electronpermeable material having a high- 1y oriented and regular atomicstructure between said electron gun and said recording medium saidwindow passing said electron beam with substantially no defocusingthereof.

2. The apparatus acoerding to claim 1 wherein said window is formed ofpyrolytic graphite.

3. The apparatus according to claim 1 wherein said window is less than500 Angstroms in thickness.

4. Apparatus for producing phase diffraction gratings comprising animage receiving surface capable of de formation when subjected toelectrostatic charge, means for directing a stream of electrons at saidimage receiving surface for producing said electrostatic charge thereon,and a thin layer of pyrolytic graphite between said electron beamproducing means and said image receiving surface through which saidstream of electrons passes without appreciable loss of coherence.

5. Radiant energy apparatus comprising an enclosure, a source ofdirected radiant energy within said enclosure, and an exit window ofthin pyrolytic graphite included as a part of said enclosure throughwhich said radiant energy is transmitted with substantially no change indirection thereof.

6. Radiant energy apparatus comprising first and second regions closedto one another except for a small aperture, means for producing a beamof radiant energy in a first of said regions directed towards a secondof said regions through said aperture, and a window of thin crystallinematerial having a highly oriented and regular atomic structure betweensaid first and second regions disposed across said aperture and throughwhich said beam passes with substantially no change in directionthereof.

7. In an electron beam apparatus, an electron gun comprising a pluralityof electrodes including a cathode and an anode electrode having a smallaperture for passing said electron beam, and a pyrolytic graphite Windowdisposed across said aperture in said anode electrode, said windowpassing said electron beam with substantially no defocusing thereof.

8. In an electron beam apparatus, an electron gun comprising a pluralityof electrodes including a cathode and an anode electrode having a smallaperture for passing said electron beam, a pyrolytic graphite windowdisposed across said aperture in said anode electrode, and an insulatingrefractory mounting ring between said window and said anode electrode,said window passing said electron beam with substantially no defocusingthereof.

9. Radiant energy apparatus comprising first and second differentiallyevacuated regions, means for producing an electron beam in a first ofsaid regions, a thin crystalline electron permeable material having ahighly oriented and regular atomic structure situated between saidregions, said material passing said beam with no appreciable loss ofcoherence thereof, means for focusing and deflecting said electron beamin the second of said regions, and an image receiving surface disposedwithin the second of said regions, said surface lying in the path ofsaid electron beam to enable said focusing means to focus said electronbeam upon said image receiving surface.

10. An electron permeable window for passing a stream of electrons, saidwindow being formed of a material having a regular and highly orientedatomic structure whose inter-atomic spacing is substantially differentfrom the wavelength of electrons passing therethrough, said stream ofeleptrons having a crosssectional dimension 12 which is large comparedto the atomic spacing of said atomic structure.

11. The electron permeable window according to claim 10 wherein thewindow is formed of material having an atomic structure whoseinter-atomic spacing is large compared to the wavelength of electronspassing therethrough, and which has crystallite planes orientedsubstantially perpendicularly to said stream of electrons.

12. The electron permeable window of claim 10 formed of pyrolyticgraphite.

13. A radiant-energey permeable window for use in radiant-energyprojecting apparatus, said window having a regular and highly orientedatomic structure whose inter-atomic spacing is substantially differentfrom the wavelength of radiant energy to be passed therethrough andbeing constituted of a pyrolytically deposited substance of the classconsisting of graphite, boron nitride, and tantalum oxide.

14. Radiant energy apparatus comprising first and second differentiallyevacuated chambers, a radiant energy permeable window providingcommunication between said chambers, and means for producing a beam ofenergy in a first of said chambers directed towards a second of saidchambers through said window, said window having a regular and highlyoriented atomic structure whose interatomic spacing is substantiallydifferent from the wavelength of radiant energy to be passedtherethrough and being constituted of a pyrolytically depositedsubstance of the class consisting of graphite, boron nitride andtantalum oxide, so as to pass said beam with substantially no change indirection thereof.

15. Radiant energy apparatus comprising means for producing a narrowlyfocused electron beam, means for deflecting said electron beam, and athin pyrolytic graphite window between said means for producing saidelectron beam and said means for deflecting said electron beam, saidwindow passing said electron beam, said widow passing said electron beamwith substantially no defocusing thereof.

16. Radiant energy apparatus comprising an electron gun producing anarrowly focused electron beam for forming an image, an evacuated firstchamber housing said electron gun, a second chamber adjacent said firstchamber, said second chamber having higher internal pressure than saidfirst chamber, a thin pyrolytic graphite window between said chambersthrough which said electron beam passes with substantially no defocusingthereof, and means for further focusing and deflecting said electronbeam after passage thereof through said window.

References Cited by the Examiner UNITED STATES PATENTS 1,943,109 1/1934Coolidge 313-74 2,698,928 1/1955 Pulvari 340173 2,820,168 1/1958 Stiff313-74 2,927,959 3/1960 Mast u--- 178-75 2,950,388 8/1960 White 313742,985,866 5/1961 Norton 340-173 3,099,762 7/1963 Hertz 313--74 3,113,17912/1963 Glenn 340-173 FOREIGN PATENTS 519,015 3/ 1940 Great Britain.627,063 7/1949 Great Britain.

TERRELL W. FEARS, Acting Primary Examiner.

IRVING SRAGOW, BERNARD KONICK, Examiners,

J. F. BREIMAYER, M. K. KIRK, Assistant Examiners.

4. APPARATUS FOR PRODUCING PHASE DIFFRACTION GRATINGS COMPRISING ANIMAGE RECEIVING SURFACE CAPABLE OF DEFORMATION WHEN SUBJECTED TOELECTROSTATIC CHARGE, MEANS FOR DIRECTING A STREAM OF ELECTRONS OF SAIDIMAGE RECEIVING SURFACE FOR PRODUCING SAID ELECTROSTATIC CHARGE THEREON,AND A THIN LAYER OF PYROLYTIC GRAPHITE BETWEEN